The neuroendocrine regulaton of pituitary activity and sexual behavior in the male and female rat differ, and these functional differences are established perinatally under the influence of testicular androgen. This research program is concerned with the identification of the neural basis of these functional sex differences in the adult and in the elucidation of their perinatal development. Recently we have discovered a marked morphological correlate of the process of sexual differentiation: the Sexually Dimorphic Nucleus (SDN) of the preoptic area. The SDN has been subjected to cellular analysis which has shown that the SDN of the male contains more neurons than that of the female and study of its development (in terms of volume) suggest that the morphological sex difference arises during the period of differentiation. We are in the process of determining the "birthdate" of the neurons of the SDN and plan to establish whether the perinatal hormone environment alters neuronal mitotic or migratory activity and/or survival to produce the adult sexual dimorphism. The capacity of the neurons of the SDN to take up and retain labelled steroids will be determined autoradiographically and compared with that of neurons elsewhere in the brain. Small lesions of the SDN will be produced in order to identify possible specific function(s) of this nucleus. Anatomical input to the SDN will be studied electrophysiologically, and local changes in neurotransmitter activity in the SDN monitored following hormone treatment in the adult. This broad attack on a specific sexually dimorphic region of the brain is expected to provide further insight into the mechanisms of the action of gonadal steroids on the developing and adult brain.